Since 1987 when C. W. Tang et al. of Kodak demonstrated high illumination light emission of an organic light emitting device (Appl. Phys. Lett., Vol., 51, page 913, 1987), development of materials for organic light emitting devices and improvement of device structure have been rapidly made. Recently, organic light emitting devices have gone into practical use firstly in displays for car audio sets, cellular phones and the like. Currently, to put such organic electroluminescent (EL) devices to a wider range of uses, development of materials for improving the light emission efficiency and durability, or development for applying them to full color displays are being actively made. In particular, on considering the use wide-spreading to the medium- or large-size panel or illumination, the high luminance must be more intensified by improving the light emission efficiency.
As such light emitting materials, metal complexes such as aluminum quinolinium complexes (Alq3), which has good light emission efficiency so that light emission intensity is high, have been commonly used. For forming such low molecular weight materials into a light emitting layer of an organic light emitting device, vacuum deposition and the like techniques have been used. This has been considered to be a demerit in the production process of the organic light emitting devices. Nature, Vol. 397, page 121 (1999) discloses that π-electron conjugated polymers such as poly(paraphenylene-vinyleneg) (PPV) and derivatives thereof (MEH-PPV) can serve as a light emitting material. These polymers have begun to be partially used as a backlight of a clock and the like. These polymer materials have not only a merit in production process because they can be formed into films by a casting method but also a merit in an increased durability as compared with low molecular weight light emitting materials. However, they have a demerit in that they have low light emission efficiency as compared with the low molecular weight light emitting materials.
In the light emitting materials as mentioned above, use is made of light emission from an excited singlet state, that is, fluorescence. According to the description at page 58 of Monthly Display, October 1998, Separate Volume “Organic EL Display”, the upper limit of the internal quantum efficiency of light emission in an organic EL has been considered to be 25% from the ratio of an excited singlet state to an excited triplet state generated by electric excitation being 1:3.
On the contrary, by using an iridium complex emitted by phosphorescence from an excited triplet state, M. A. Baldo et al. indicated that it is possible to obtain an external quantum efficiency of 7.5% (this corresponding to an internal quantum efficiency of 37.5% assuming that the light out-coupling efficiency is 20%), which is higher than the external quantum efficiency of 5%, which has conventionally been considered to be the uppermost value (Appl. Phys. Lett., Vol. 75, page 4 (1999), WO00/70655). However, materials that stably emit phosphorescence at room temperature as the iridium complex as used in the prior art cited here are extremely rare so that freedom of selection of materials is narrow and they must be doped to a specified host compound when actually used. Therefore, the conventional materials have the disadvantage that selection of materials that satisfy the specification of a display is extremely difficult to make.
On the other hand, M. A. Baldo et al., also indicated that a relatively good light emission efficiency can be obtained by using an iridium complex as a sensitizer, transferring energy from an excited triplet state of the complex to an excited singlet state of a fluorescent dye, and finally emitting fluorescence from the excited singlet state of the fluorescent dye (Nature, Vol. 403, page 750 (2000)). This method has the advantage in that from a number of fluorescent dyes, one that is suitable for the purpose can be selected and used. However, this process has the great disadvantage in that it has low quantum efficiency of light emission in principle since it involves a spin-forbidden process of transferring energy from an excited triplet state of a sensitizer to an excited singlet state of a fluorescent dye.
Next, concerning the mass production method of panels, conventionally a vacuum deposition method has been used. However, the vacuum deposition method cannot be always suitable for mass production of panels having large areas since they have the problems in that they require a vacuum equipment and it becomes more difficult to forming an organic thin film so as to have a uniform thickness according as the film has a larger area.
In order to improve the disadvantage, a production method using a polymer light emitting material, that is, an ink jet method and a printing method have been developed as methods that facilitate the production of large area products. In particular, the printing method can continuously form a long film so that it is excellent in the production of large area products and in mass productivity.
As described above, in order to obtain an organic light emitting device having a high light emission efficiency and having a large area, a phosphorescent polymer material is required. As such phosphorescent polymer materials, polymers incorporating ruthenium complexes in the main chains or side chains of the polymers are known (Ng, P. K. et al., Polymer Preprints., Vol. 40(2), page 1212 (1999)). These compounds are ionic compounds and application of a voltage thereto causes electrochemical light emission due to the oxidation reduction reaction at the electrodes. The electrochemical light emission shows an extremely slow response in the order of minutes so that they are unsuitable for usual display panels.
In addition, there is a composition of poly(N-vinylcarbazole) mixed or dispersed with a low molecular weight phosphorescent compound, an iridium complex, although it cannot be said to be a polymer material in a strict sense (P. J. Djurovich et al., Polymer Preprints, Vol. 41(1), page 770 (2000)). However, there is a possibility that this material is poor in heat stability as compared with homogeneous polymer materials and tend to cause phase separation or segregation.
Japanese Patent Application Laid-open No. 2001-181616, Japanese Patent Application Laid-open No. 2001-181617, and Japanese Patent Application Laid-open No. 2001-247859 disclose organic light emitting materials composed of phosphorescent ortho-metallized palladium complex, ortho-metallized platinum complex and ortho-metallized iridium complex, respectively, and also refer to polymer compounds having these complex structures as repeating units. However, these publications fail to disclose specific exemplification of the structure and method of preparing polymers that are necessary for forming polymers by binding the complex structures as the repeating units disclosed in the publications and discloses no practically usable phosphorescent polymer compounds.